It’s really in the seventh century B.C.E., when the small kingdom of Lydia introduced the world’s first standardized metal coins, that you start to see money being used in a recognizable way. Located in what is now Turkey, Lydia sat on the cusp between the Mediterranean and the Near East, and commerce with foreign travelers was common. And that, it turns out, is just the kind of situation in which money is quite useful.

To understand why, imagine doing a trade in the absence of money-that is, through barter. (Let’s leave aside the fact that no society has ever relied solely or even largely on barter; it’s still an instructive concept.) The chief problem with barter is what economist William Stanley Jevons called the “double coincidence of wants.” Say you have a bunch of bananas and would like a pair of shoes; it’s not enough to find someone who has some shoes or someone who wants some bananas. To make the trade, you need to find someone who has shoes he’s willing to trade and wants bananas. That’s a tough task.

With a common currency, though, the task becomes easy: You just sell your bananas to someone in exchange for money, with which you then buy shoes from someone else. And if, as in Lydia, you have foreigners from whom you’d like to buy or to whom you’d like to sell, having a common medium of exchange is obviously valuable. That is, money is especially useful when dealing with people you don’t know and may never see again.

In this same vein, this reply on Reddit to “Where has all the money in the world gone?” is also worth a read.

The thing to remember is that all throughout, from the initial trade to this central-banking system, all of this money is debt. It is IOUs, except instead of being an IOU that says “Kancho_Ninja will give one bushel of apples to the bearer of this bond in October”, it says “Anyone in town will give you anything worth one bushel of apples in trade.”

The money is not an actual thing that you can eat or wear or build a house with, it’s an IOU that is redeemable anywhere, for anything, from anyone. It is a promise to pay equivalent value at some time in the future, except the holder of the money can call on anybody at all to fulfill that promise — they don’t have to go back to the original promiser.